This invention relates to jet drop copying systems of the general type disclosed in Paranjpe et al U.S. Pat. No. 4,112,469. In the Paranjpe patent there is disclosed a scanning optical system which scans an original document and images a portion thereof upon the face of a photodetector array. The photodetectors within the photodetector array are arranged in correspondence with the arrangement an array of orifices in a jet drop print head. The jet drop print head projects drops of printing liquid from each of the orifices, and these drops are directed against a printing sheet mounted on a rotating drum. As the drum rotates, the print head prints a series of spaced tracks. Means for provided for advancing the print head in the axial direction in synchronism with the drum rotation, so that the printing tracks interlace in helical fashion.
As also taught in the Paranjpe patent, the photodetector array and the scanning optics are mounted on a table which translates in synchronism with the axial advance of the print head. The optics include a first lens which views a portion of the original document and collimates the light received from the portion which is viewed. The collimated light is directed against a scanning mirror, and from there it is reflected toward a second lens for imaging. The face of the photodetector array is positioned within the image plane of the second lens. The scanning mirror is rotated in synchronism with the rotation of the printing drum, so that there is continuous correspondence between the document spots being viewed by the photosensors and the printing sheet spots which are being printed by the jet drop print head.
The print head which is disclosed in the Paranjpe patent is considerably shorter than the printing drum, so that it requires a great number of drum rotations and a considerable amount of printing head movement to complete the printing of one copy. This severely limits the speed capability of the system. In this regard it should be noted that the system speed is limited not only by the length of the print head but also by the basic nature of the helical interlacing technique. Such interlacing requires considerable movement of the printing head, and as the head grows longer more movement is required for completion of the interlacing. This requirement for printing head movement applies equally to the table which moves the optical system, and thus it is seen that a different type of scanning technique must be used for systems required to operate at speeds much higher than those contemplated by Paranjpe et al.
For very high speed printing as required, for instance, in newspaper printing, it is feasible to use staggered rows of jets and photodetectors as taught in Taylor et al RE28219. This type of system provides solid printing coverage in a single pass, but it ialso requires a very large number of jets, photodetectors, and associated electronics. Such a system is inherently very expensive and provides printing speed capabilities far in excess of those required in applications such as office duplicating, wherein the speed requirement is in the order of about two printed pages per second.
Cahill et al U.S. Pat. No. 3,689,693 teaches a banding type of printing technique wherein a plurality of print heads cooperatively print an image upon a print sheet mounted on a rotating drum. The print heads are advanced axially while the drum rotates, and the axial advance is adjusted such that the printing head arrangement moves a distance equal to the width of one printing track during one drum rotation. Printing progresses until solid coverage is obtained. This technique requires relatively less movement than an interlacing technique of the type disclosed in Paranjpe et al. However, the arrangement as disclosed in Cahill et al scans, stores, and formats printing control information off line from the printing operation. No means are disclosed for "on line" document copying, and the multiple print head arrangement as disclosed is severely limited as to density of printing jets. Thus the system is quite slow and unsuited for use as an office copier.
Another important consideration, involved in the design of a suitable jet drop copying system has to do with the matter of document illumination. For higher speed operation of the type herein contemplated, the illumination requirements become quite severe. This is due to system noise which is inherent in the operation of a photodetector array. In order to produce a noise free signal for switching control of a jet, it is necessary to provide the photodetectors with incident light which is either long in duration or high in magnitude. For the very short observation times available in a high speed copier, a simple arrangement of illumination lamps, as shown in the above mentioned Paranjpe et al patent, has been found to be inadequate. A similar problem exists in high speed electrophotographic copiers. One solution, as taught in Libby U.S. Pat. No. 3,694,070, involves providing a pair of illumination lamps together with a pair of scanning reflectors that focus the light only on the area of the original document that is actually being optically scanned by the system.